1. Field of the Invention
The present invention relates to a navigation system mounted on a vehicle for providing support to a vehicle driver such as route guide for the vehicle driver, and to a gyroscopic device which is used in the navigation system, or similar systems, as bearing detecting means for detecting changes in progress direction.
2. Description of the Related Art
Such a navigation system 1 of which hardware construction is shown in FIG. 14 is conventionally mounted on the vehicle. A processing apparatus 2 of the navigation system 1 detects the present position of the vehicle on the basis of the information detected by a sensor 3, and then a present position of the vehicle is displayed on a display 5 on the basis of the data stored in a storage apparatus 4. A mass storage apparatus having a replaceable recording medium, such as a CD-ROM or an IC card, or a stationary mass storage apparatus, such as a hard disk, is used as the storage apparatus 4. A CRT, LCD, or the like is used as the display 5. The processing apparatus 2 includes a CPU 6, a main memory 7, and the like. The main memory 7 includes a RAM and a ROM, wherein the ROM stores preset programs. The navigation system 1 is capable of detecting an absolute position and an absolute bearing of the vehicle by receiving electric wave signals from an artificial satellite 8 constituting a GPS (Global Positioning System).
FIG. 15 shows a functional construction of the navigation system 1 of FIG. 14. The sensor 3 includes a bearing sensor 10 for detecting a moving direction of the vehicle, a distance sensor 11 for detecting a distance traveled by the vehicle, a GPS apparatus 12 for detecting the absolute position and the absolute bearing by receiving electric waves from the artificial satellite 8 of the GPS. The signals detected by the sensor 3 are processed by a sensor signal processing portion 13, realized by the program operation of the CPU 6, as signals representing the moving amount and the moving direction of the vehicle. A map matching process, wherein the vehicle position is applied on a road which is read from road data 15 through a road data reading portion 16 in accordance with the moving amount and the moving direction from the sensor signal processing portion 13, is effected in a map match processing portion 14 which is realized by the program operation of the CPU 6. In the road data 15, roads are stored with successive formation of the links among straight line sections. The vehicle position presumed on the road by the map match processing portion 14 is displayed on the display 5 of a display apparatus or similar device.
In the detection of the absolute position and the absolute bearing by the GPS apparatus 12, the present position and the progress direction of the vehicle are presumed by a self-supporting navigation based on detection signals from the bearing sensor 10 and the distance sensor 11 realized by a vibration gyro and the like. This is because comparatively large errors may be included in the detected absolute position and absolute bearing even if these could be anyhow detected due to the fact that the receiving condition of the electric waves from the artificial satellite 8 is not always satisfactory. In the estimation of the vehicle bearing by the self-supporting navigation, since changes in the running direction of the vehicle are accumulated by a vibration gyro, measurement errors of the vibration gyro and the like are accumulated, which causes a deterioration in the reliability of the bearing detection. The vehicle direction and position presumed by the self-supporting navigation is corrected on the basis of the absolute position and absolute bearing detected by the GPS apparatus 12, or is corrected by conducting feedback of the corrected vehicle position or the link bearing which is the bearing of the road during the map matching process to the road by the map match processing portion 14. Also, a method wherein the current running position of the vehicle is directly inputted, thereby to correct the vehicle position, is also in use.
In the conventional navigation system, the position and progress direction of the vehicle can be detected by plural methods to make up for the loss of the respective methods. For example, Japanese Unexamined Patent Publication JP-A 3-23490 discloses a prior art relating to a detection of the progress direction of a vehicle as an absolute bearing, by means of an earth magnetic sensor and a GPS apparatus, to detect the abnormality of output data from the earth magnetic sensor.
Also, in the navigation system, as described above, both a method of presuming a relative present position by the self-supporting navigation on the basis of a change in a running direction and a running distance of the vehicle and a method of absolutely detecting the present position and progress direction of the vehicle by the GPS are used. The condition of receiving the electric waves in the GPS is not always good, and errors in the absolute position and the absolute bearing sometimes become large even when they are detectable. To detect the relative bearing, a gyroscopic device using a vibration gyro and so on is preferably used, and bearing detection is always enabled. The bearing detection using the gyro sensor is effected by multiplying the difference between an output value of the gyro sensor and a gyro offset value which is outputted when the rotation is not given in the running direction of the vehicle, by a gyro sensitivity coefficient, and then by integrating the result. However, since the offset value changes with time due to temperature drifts or the like, proper correction is required. This is particularly the case at the time of starting since deviations by several ten degrees in five minutes may be caused.
The prior art concerning the correction with respect to the temperature drift or the like of the gyro sensor is disclosed in Japanese Unexamined Patent Publication JP-A 3-95407, Japanese Unexamined Patent Publication JP-A 4-235310, Japanese Unexamined Patent Publication JP-A 7-218276 and the like. The Japanese Unexamined Patent Publication JP-A 3-95407 discloses an art of offset correction considering the amount of change on the assumption that the drift component changes successively with time. Japanese Unexamined Patent Publication JP-A 4-235310 discloses an art of calculating a necessary offset value in a short time by increasing the sampling frequency for calculating the offset correction value at the time of a temperature rise of the apparatus. Japanese Unexamined Patent Publication JP-A 7-218276 discloses an art of avoiding a wrong correction in estimation of the correction amount for an offset bias amount during the stop of the vehicle to effect the offset correction.
In the case where the vehicle bearing is corrected with a combined use of plural methods such as a detection of an absolute bearing, map matching in the conventional navigation system, and the like, the following problems are encountered:
(1) When a link bearing and an absolute bearing of a map-matched road are largely different from each other, the feedback thereof is considered to be contradictory to each other. This is particularly the case when the map matching is erroneous, thus there is a possibility that the convergence on a correct bearing may be effected with great difficulties. PA1 (2) When the vehicle bearing and link bearing obtained by the vibration gyro or the like are largely different from each other, there is a high possibility that the map matching is erroneous, and the error of the vehicle bearing potentially becomes larger in the case where correction to the link bearing obtained by the map matching is carried out. PA1 (3) When the vehicle bearing and absolute bearing obtained by the vibration gyro or the like are largely different from each other, the error of the vehicle bearing is large, and there is a high possibility of presumption of a wrong road whether or not the map matching was effected. Accordingly there is a possibility that the error of the vehicle bearing does not converge when the bearing correction to link bearing is effected after the map matching. PA1 (4) When the correction bearing by the absolute bearing and the correction bearing by the link bearing are the same in direction, there is a possibility that only the vehicle bearing is erroneous even when a bearing difference with respect to the vehicle bearing is large, and accordingly, there is a possibility of being late in convergence of the vehicle bearing in the case where the correction is not effected for a reason that the bearing difference with respect to the vehicle bearing is large. PA1 (5) When the straight advance judgment or the like is effected using the vehicle bearing obtained by the vibration gyro, the dispersion of the bearing becomes large under the influences of the bearing correction, and accordingly, there is a possibility that the bearing data cannot be obtained even in the case of straight advance. PA1 (6) When the bearing correction is effected by the absolute bearing and vehicle bearing, there is a possibility of effecting a wrong correction under the influence of an error component in the case where the correction is effected by only one sample value. PA1 history calculating means for calculating a running history on the basis of the vehicle position and the bearing showing the progressive direction of the vehicle; PA1 position correcting means for calculating similarity between the running history of the vehicle calculated by the history calculating means and road data stored in advance, to thereby correct the vehicle position on a road where the similarity is large; PA1 bearing correcting means for correcting the bearing showing the progressive direction of the vehicle to a bearing of a road on which the vehicle position is corrected by the position correcting means, PA1 wherein the correction of the vehicle position by the position correcting means is carried out while correction of the bearing showing the progressive direction of the vehicle to the bearing of the road on which the vehicle position is corrected by the position correcting means is not carried out, when a predetermined bearing difference condition is established. PA1 history storage means for sequentially capturing a bearing showing a progressive direction of a vehicle and storing a history of the bearing; and PA1 straight advance judging means for determining a straight advance when a change in bearing during a predetermined period is within a predetermined range, referring to the history storage means, PA1 wherein the correction bearing calculating means calculates a correction bearing, to which the bearing showing the progressive direction of the vehicle is to be corrected, separately from a bearing showing a presumed progressive direction when the straight advance is determined by the straight advance judging means. PA1 clocking means for clocking a time elapsed; PA1 offset detecting means for detecting offset components from an output of the gyro sensor under a predetermined condition where the progressive direction is judged as being stable; PA1 correction value calculating means for calculating an offset correction value, the offset correction value being for correcting the offset components detected by the offset detecting means; and PA1 offset correcting means for correcting the output from the gyro sensor by using the offset correction value calculated by the correction value detecting means, PA1 wherein the offset correcting means makes a degree of the correction in a period from a starting-up of the power source to a predetermined time, larger than degrees of the correction in other periods, referring to the clocking means.
The gyroscopic device, such as a vibration gyro, is likely to generate a temperature drift caused by the rise in the atmospheric temperature due to heating of a gyro vibrator itself, and heating of peripheral circuits for signal processing, thus resulting in that the offset change becomes so large as it cannot be neglected when high precision is requested. Therefore, as described in the prior art of Japanese Unexamined Patent Publication JP-A 4-235310, even if an offset correction is effected in a short time immediately after the temperature rise of the apparatus, the subsequent temperature drifts cannot be prevented.
To correct the offset from the temperature drift, it is required to actually detect the offset component to effect proper offset correction. Generally, the detection of the offset value is effected during the stop time or in the straight advance time when the change in progress direction of the vehicle is smaller. However, when driving on a mountain road or a highway, since the vehicle continues running for a long time and a period when the vehicle advances straight is relatively short, the condition to effect the correction of the offset value is not satisfied for a long time, so that there is a possibility of the true offset value being largely different due to temperature drifts.